In some metal jewelry applications, such as in preparing a ring, a lighter than usual color of gold is desirable. One technique to lighten the color is often termed “bleaching”. There are two elements, nickel (Ni) and palladium (Pd), which are commonly used to bleach the color of gold when preparing white gold alloys for jewelry applications. Ni and Pd are preferably used because both metals exhibit bright gray color, and both have the same face-centered cubic crystalline structures as other major alloying elements such as gold, silver and copper.
In the past decade or so, concern has grown with regard to the inclusion of Ni in jewelry. See, for example, Roy Rushforth, “Don't Let Nickel Get Under Your Skin—The European Experience”, The Santa Fe Symposium on Jewelry Manufacturing Technology, 2000, 281-301. The increased concern about nickel as a cause of allergy has led to the development of Ni-free Pd-containing white golds alloys. See, for example, Valerio Faccenda, “On Nickel White Gold Alloys: Problems And Possibilities”, The Santa Fe Symposium on Jewelry Manufacturing Technology, 2000, 71-88). The increased concern about nickel as a cause of allergy has led to the development of Ni-free Pd-containing white golds alloys. By definition, 18 karat gold alloys must contain 75% gold by weight (w %) minimum to be considered 18 karat, leaving only 25 w % for all other elements, including other major elements such as silver (Ag), copper (Cu) and, in the case of white gold, Pd. In Ni-free, Pd-containing alloys, Pd tends to be in the approximate range of 6 w %-15 w %. The increasing price of precious metals including Pd makes it desirable for Ni-free white gold alloys to contain as little Pd as possible while still retaining the color and durability benefits of Pd-containing white gold alloys. However, a too-limited percentage of Pd, especially when combined with the elimination of Ni in the alloy, leads to the deterioration of the white color overall.
The whiteness of the color of gold alloys can be graded using the yellowness index (YI) as described in “White Gold Alloys: Colour Measurement and Grading” by Steven Henderson and Dippal Manchanda in Gold Bulletin, 2005, vol. 38, issue 2, pp. 55-67. According to this reference there are three grades of alloys which can be considered white in color:
TABLE 1Grade:WhitenessYIRhodium platingGrade 1:Good WhiteYI < 19.0Not necessaryGrade 2:Reasonable White19.0 < YI < 24.5OptionalGrade 3:Off-White24.5 < YI < 32.0Recommended
Examples:
One commercially available 18K alloy that contains 15% palladium shows a good white color with YI=16.3 as listed in Table 2.
TABLE 2Alloy% PdYICommercial1516.3Vincent, Table I, Alloy 11418.3Vincent, Table I, Alloy 51319.7Vincent, Table I, Alloy 131219.7Vincent, Table II, Alloy 4730.1Vincent, Table II, Alloy 12631.8
U.S. Pat. No. 6,787,102 to Vincent (“Vincent”) teaches the formation of various alloys. Table 2 also lists YI for some of these alloys.
For comparison, a commercially available 18k yellow alloy containing no palladium has YI=48.5.
Alloys with YI>32.0 cannot be called white.
Further, the whiteness in some of the alloys of Vincent, particularly the alloy with 7% palladium shown in Table 2, is close to the borderline in what can be classified as white. This can be a concern because color properties of an alloy may vary from batch to batch due to a variety of factors, and when a color is close to a border, at times the color variability may cause a particular formulation to fall outside the white range. As a result, it is desirable to form an alloy which provides better assurance of being considered white and meets other characteristics of hardenability and cost while remaining nickel free.
Age hardening is beneficial because such alloys can be easily worked and formed in the annealed condition into the jewelry article, and then the finished jewelry article can be age hardened. Age hardening of jewelry applications shows improved durability and polish. Age hardening also allows for the manufacturing of light weight, thin walled, and reduced cross section jewelry articles without compromising the strength and durability. This process also provides additional savings on the material cost.